The present invention relates to a digital frequency and phase comparator for detecting a frequency of a digital signal and a phase thereof relative to a reference signal.
Generally, a digital frequency and phase comparator for comparing a frequency and a phase of an input pulse signal with those of a reference pulse signal to detect the differences in frequency and in phase therebetween is widely employed in the case where the input pulse signal is synchronized with the reference pulse signal, and various digital frequency phase comparators have been proposed.
FIG. 1 shows a phase comparator which has been widely used in a conventional digital frequency and phase comparator. In this drawing, a phase comparing portion 10 (enclosed by a chain line) is constituted by inverters 11 to 14, flip-flops 15 and 16, and NAND gates 17. Reference numeral 20 designates a current changeover switching circuit (also enclosed by a chain line). A reference pulse signal S and an input pulse signal P are applied to input terminals IN.sub.1 and IN.sub.2, respectively, of the phase comparing portion 10, and a phase difference signal D representing the phase difference between the input and reference pulse signals is produced at an output terminal OUT of the phase comparing portion 10.
In this arrangement, in normal operations, the phase difference signal D at the output terminal OUT has a pulse width corresponding to a phase difference, that is the difference in timing between rising edges of the reference and input pulse signals S and P, as shown in FIG. 2. Such a phase comparator is well known, for example, from U.S. Pat. No. 3,610,954, and therefore a further detailed description as to the operation thereof is omitted. A typical product implementing such a phase comparator is integrated circuit type MC-4044 manufactured by Motorola Inc.
In such a phase comparator shown in FIG. 1, there is a possibility that the output phase difference signal D will be inverted in polarity in the case where noise due to drop-out or the like occurs. Such a polarity inversion, as illustrated in FIG. 3, in the phase difference signal D occurs when a false input signal b.sub.1 is caused by noise due to drop-out or the like in the input pulse signal. This makes it impossible to provide correct information as to the phase difference.
Further, the conventional digital frequency and phase comparator has a saturated characteristic, as shown in FIG. 4, when the frequency falls in a range in which the phase difference exceeds a predetermined value such as .+-..pi. or the like.
Thus, in the conventional digital frequency and phase comparator, there occur such problems as the phase difference signal, being inverted in polarity by a false input signal due to drop-out or the like, causing an erroneous controlling operation, and that the phase comparison characteristic, saturating in a frequency band where the phase difference exceeds a predetermined value, making it difficult to obtain a good transient response. Further, with the conventional phase comparator, it is impossible to detect the phase and frequency differences independently of each other.